Five UC San Diego academic leaders are among three dozen of the University of California’s highest-paid executives to threaten a lawsuit unless they receive a boost in pension benefits they say was promised to them.

Their demand is outlined in a Dec. 9 letter to the UC Board of Regents obtained by the San Francisco Chronicle.

“We believe it is the university’s legal, moral and ethical obligation,” to increase the benefits for employees earning more than $245,000, the executives wrote. “Failure to do so will likely result in a costly and unsuccessful legal confrontation.”

The university system estimates that the pension hike would add $5.5 million yearly to its $21.6 billion unfunded pension liability. In addition to that, UC would be on the hook for a one-time $51 million cost to make the increases retroactive to 2007.

To date, pensions are calculated as a percentage of the average of the last three years of pay, capped at $245,000. For a 30-year employee, the maximum pension is therefore $183,750 — whether their final salary is $245,000 or much more.

If the cap is lifted, someone making $700,000 could make a pension of $525,000.

The executives who signed the letter say the regents agreed in 1999 to bump pensions once the Internal Revenue Service allowed them to lift the $245,000 cap. The IRS did just that in 2007 over the objections of some taxpayer groups.

“Many individuals made a decision to come to the university based on the benefits (offered) when they were being recruited,” said Robert Sullivan, dean of UC San Diego’s Rady School of Management, who signed the letter. “The larger issue going forward is, can anyone who’s being recruited trust the salary package that’s being offered?”

But UC officials said the proposal to increase pensions if the IRS allowed it was never a guarantee, that it was always conditioned on several levels of final approval.

“It was never that explicit. The pension increase always required approval from the president...,” Steve Montiel, spokesman for the UC president’s office, said on Wednesday.

University of California president Mark Yudof has opposed the pension increases amid controversy over the UC’s sizable shortfall in money available to pay pensions promised to existing employees. At the same time, he is aware that UC campuses need the ability to offer salary packages that allow them to recruit top academics, Montiel said.

“The president is trying to thread the needle between competitive salaries needed to attract the best talent and fiscal prudence,” Montiel said.